In the electromagnetic spectrum, a 0.1 to 10 THz (1 THz: 1012 Hz) band is defined as a terahertz wave band. In particular, a 0.1 to 3 THz band is a band in which rotational resonant frequencies of a great variety of molecules are present, and molecule detection or the like can be performed in a non-destructive manner, a non-open manner, or a non-contact manner by exploiting the terahertz wave band characteristics of such molecules. Such terahertz wave technology enables the provision of new concept future core technologies, which have not yet been introduced to date, in the fields of medical treatment, medicine, agriculture and food, environmental measurement, biology, communication, non-destructive investigation, advanced material evaluation, etc., and very fierce competition has been conducted in the development of related core technology.
In terahertz wave technology, since the energy of photons in the terahertz wave band is as very low as several meV, it scarcely influences a human body and is recognized as a core technology for realizing a human-centered ubiquitous society, and thus it is predicted that the demand for the terahertz wave technology will be rapidly increased. However, technology that can simultaneously satisfy real-time properties, portability, low price, broadband, etc. has not yet been developed. However, thanks to the continuous improvement of technical skills, various presentations related to the utilization of terahertz spectroscopy and imaging fields have currently been made. Unlike terahertz imaging fields in which a high-power wave source and a high-sensitive array detector are essentially adopted, a broadband terahertz wave source has been settled as core technology for the system in terahertz spectroscopy.
The broadband terahertz system that has been most widely used until recently is a terahertz (THz)-Time Domain Spectroscopy (TDS) system for radiating femtosecond ultrashort pulse laser light to a semiconductor having an ultrahigh response speed and generating terahertz waves, as can be seen in FIG. 1. Since a broadband terahertz spectroscopy system composed of a femtosecond high-output pulse laser and a Photoconductive Antenna (PCA) may be implemented to relatively easily provide high Signal-to-Noise Ratio (SNR) and broadband characteristics, it is the first commercialized system. The THz-TDS system of FIG. 1 is configured such that a femtosecond light pulse 10 from a single femtosecond laser 22, reflected by a mirror M1, is split into two femtosecond light pulses by a beam splitter BS. Of the two femtosecond light pulses, one femtosecond light pulse is reflected by a mirror M2 to excite a THz emitter 12, and the other femtosecond light pulse sequentially passes through an optical delay unit DL and mirrors M3 and M4 and is input to a THz detector 18. Two off-axis parabolic mirrors 14 disposed downstream of the THz emitter 12 focus THz beams from the THz emitter 12 on a sample 16, and two off-axis parabolic mirrors 20 condense the THz beams having passed through the sample 16 and focus the THz beams on the THz detector 18. At a position where paths of the left and right laser beams are exactly identical to each other, the maximum value of terahertz signals can be measured. Methods of measuring terahertz signals are performed using a sampling method based on a difference between optical paths by gradually changing the optical path of a right laser beam using the optical delay unit DL.
However, since the above-described THz-TDS system is implemented as a dedicated and complicated optical system including the femtosecond laser 22, the optical delay unit DL, etc., it is very expensive and has a large system size. In particular, the THz-TDS system of FIG. 1 has difficulty in real-time measurement due to time required for optical delay and time required to process a Fast Fourier Transform (FFT) on measured time domain signals. Such problems have been recognized as factors to be solved for the purpose of maximizing industrial utilization.
Recently, in addition to the THz-TDS system which is a scheme for generating pulsed broadband terahertz waves, a lot of effort to develop THz-Frequency Domain Spectroscopy (FDS) systems for generating continuous waves shown in FIG. 2 is currently being made. It is possible to provide high frequency resolution based on a continuous wave scheme and to develop an inexpensive, broadband, and micro-size system by utilizing two independent high-power semiconductor lasers, so that a terahertz spectroscopy system that can be applied to various fields can be developed, and a plurality of institutions are competitively developing related technologies. However, instances that are substantially applied in detail to systems are not presented due to very bad photoelectric conversion efficiency of the continuous wave scheme.
The THz-TDS system which is a pulsed broadband terahertz wave generation system shown in FIG. 1 generally uses a titanium sapphire (Ti: Sapphire) laser which is a femtosecond ultrashort pulse laser, and is implemented using a PCA which is a terahertz wave generator based on femtosecond light excitation, that is, an ultrahigh frequency photoelectric converter (optical-to-electrical converter). The center oscillation wavelength of a commercialized Ti: sapphire laser absorbs 800 nm, and the commercialized Ti: Sapphire laser uses low-temperature grown GaAs, which has a very short carrier lifetime, as a PCA active material. In the configuration of the terahertz spectroscopy system, it is required to adopt a material that efficiently absorbs an excitation light source or has a femtosecond-level carrier lifetime essential for broadband characteristics. In spite of the same scheme, an FDS system which is a continuous wave oscillation scheme shown in FIG. 2, unlike the pulsed TDS system shown in FIG. 1, has been developed and is in competition with the TDS system.
Compared to FIG. 1, a difference with FIG. 2 is that an excitation light source utilizes beating formed by two wavelengths λ1 and λ2 of very stable high-power distributed feedback lasers DFB1 24 and DFB2 26, rather than femtosecond lasers. The terahertz wave generation scheme except for the light source is similar to that of the THz-TDS system of FIG. 1. In the case of a PCA that is an ultrahigh frequency photoelectric converter for THz-TDS, broadband terahertz waves can be easily generated using a rectangular light excitation area having a size of several micro meters and a very simple dipole antenna owing to the high peak value of an ultrashort pulse laser. In contrast, the THz-FDS system of FIG. 2 is generally referred to as a photomixer instead of a PCA because terahertz waves having a frequency corresponding to a difference between two wavelengths are generated. For the development of a photomixer 30 for generating a continuous wave other than a pulsed wave, a finger-shaped interdigitated (IDT) pattern shown in FIG. 4 is utilized by exploiting a continuously oscillating light source at several tens of mW, unlike the femtosecond laser having a very high peak value. By utilizing the IDT pattern, it is possible to generate broadband terahertz waves even at relatively low input optical power although the optical power is easily saturated and is dependent on the polarization of incident light, and thus the photomixer has been widely utilized.
FIG. 3 is a schematic diagram showing a typical photomixer. A photomixer 30, which is a device for generating broadband terahertz waves, includes a photoconductive switch (PCS) 32 made of a material, the reaction speed of which is as very high as picoseconds (10−12), and configured to allow electric current to flow therethrough when light is radiated; and antennas 34 configured to acquire the gain of the generated terahertz waves in one direction. Meanwhile, referring to FIG. 4, it can be seen that the antennas 34 are formed on opposite sides of the PCS 32 of the photomixer 30, with the PCS 32 interposed between the antennas 34.
Korean Patent Application Publication No. 2011-0069453 (entitled “Photomixer module and method of generating terahertz waves using the same”) discloses technology for increasing the intensity of excited light required to generate terahertz waves and improving the stability of a photomixer.
The photomixer module disclosed in Korean Patent Application Publication No. 2011-0069453 includes a semiconductor light amplifier configured to amplify incident laser light, and a photomixer configured to be excited by amplified laser light and configured to generate a continuous terahertz wave, wherein the semiconductor light amplifier and the photomixer are formed to be integrated into a single module.
The present invention is intended to develop a high-efficiency photomixer, which can fundamentally overcome the deterioration of characteristics due to the saturation of input light caused by the injection of a high-power excited light and a sudden increase in the temperature of the active layer of the photomixer caused by the injection of the excited light in a broadband photomixer, which is a continuous terahertz wave generation device, unlike a pulsed PCA. In particular, the background of the present invention is to develop broadband photomixer technology, which can rapidly improve broadband terahertz wave generation efficiency that was very low in a long wavelength band, and which has high efficiency and high reliability characteristics because previously well-developed parts for optical communication can be easily utilized.